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What is it about measurements in this industry?

I have a background as a trained lab scientist - Ph.D in Toxicology. Measurements need to be precise, but more importantly they need to be accurate.

Precise = How many digits of precision the value has, so 10.2983 is more precise than 10.4

Accurate = How close the reported value is to the actual value. If the actual value is 10.35 then 10.4 is more accurate than 10.2983

There are two places where measurements get me in this industry. One is the reported volume of aquariums and the other is the reported length of fish.

The manufacturers seem to measure the external dimensions of an aquarium, plug those into a calculator and report the value. The error in that method starts out fairly small for small tanks, where the glass can be fairly thin, but accuracy worsens the larger the tank, with thicker walls.

For instance, my 300DD from Marineland is said to be approximately 300 gallons. The outside dimensions are:

72" x 36" x 27"

Plug those numbers into a calculator and you get 302.96 gallons. Lets call it 303. 1% above 300 so approximate, right? Except those dimensions are the done including the bracing and frame of the tank. If I measure the actual glass (I just did this moments ago), the outer dimensions are:

71.125" x 35.125" x 26.125"

I am making an assumption that on the height that the frame is adding 7/8" because I really can't measure it directly very well. Plug those numbers into the calculator and the volume becomes 282.5 gallons. Okay, a difference of 17.5 gallons, or about 5.8% low. Still kind of approximate, depending on your definition of approximate. But that is the external dimensions. The thickness of the glass takes away from that because the water is bound by the internal dimensions of the walls, not the external. The walls of the tank are 5/8" (0.625"). Subtracting that from the above numbers gives us:

70.5" x 34.5" x 25.5"

Plug that in and you get 268.5 gallons. That is just a little more than 10% less than 300 gallons (10.5% to be more precise). I'm not an engineer, but I would have to wonder how many engineering disciplines would accept being off by 10% is good enough to call it approximately the same.

Mind you, I am not calling out Marineland on this. This is an industry-wide practice. I just happen to have one of their tanks on hand to make actual measurements.

The reason that this can be critical to know is if you even have to dose your tank with something, and you need a fairly accurate number of how much volume there is in the tank (and associated sump/refugium/etc.) to give a proper dose. If I went by the 300 gallon figure, I would be overdosing by 10%. It is also something to consider when estimating how much sand and live rock you might want, and how much the resulting tank will weigh.

The length of a fish. I understand there are at least a couple different measurements. Total Length, the more common in the industry, is supposed to be from the tip to end of tail, with the assumption that the tail is compressed along the midline of the fish. Standard length is from the tip to the base of the tail. Now, I don't claim to be real accurate when measure the length of a fish purely with my eyeballs, looking through glass into water which makes it more difficult due to index of refraction, but all I can say is that there are a lot of insecure men out there measuring fish because I have gotten a 2" Pterois radiata that could not have been much over 1" even with the tail compressed. Same can be said of a lot of fish I have seen. Even the guy who does maintenance for me, who used to own a fish store, agrees. These days when I look at a stated length of a fish, like on DD, I knock off at least 20% of the stated length.